Can class loading be affected by concurrency ?

Question

I remember having read that class loading is guaranteed to happen on a single thread, ie. that static initializer blocks do not need to worry about being invoked concurrently.
I can't find any longer the source that stated that; so is anyone able to tell me if that's true (and point me to a resource about that) ?
I could find that java.lang.ClassLoader
defines a :
protected synchronized Class loadClass(String name, boolean resolve)
but I imagine a case where a classloader overrides that method without keeping the synchronization AND a class to be loaded is referenced from many parts of a multithreaded system, so it may happen that multiple thread attempt to load it.
is this a real scenario ?
TIA,
Edo

Answer 1

Hello,
Based on the JVM specifications, when a class or interface is initialized (execution of the static initializers) the "Java Virtual Machine is responsible for taking care of synchronization". The very first step of the procedure for initializing a class or interface is to "synchronize on the Class object that represents the class or interface to be initialized".
Now, the questions are:
How to interpret the specifications? Are the JVM implementations compliant?
Personally, when I write some static initializer blocks, I do NOT worry about multi-threading issues. But I may be wrong.
For instance, I write:
class Singleton {
  private static Singleton SINGLETON = new Singleton();
  public Singleton getInstance() { return SINGLETON };
}and not:
class Singleton {
  private static Singleton SINGLETON;
  static {
    synchronized(Singleton.class) {
      SINGLETON = new Singleton();
  public Singleton getInstance() { return SINGLETON };
}I hope it helps.